Question

The number of chiral centres present in [B] is ________.

(Reaction scheme as drawn: 3-methylcyclohexyl-CH(CH₃)-C\(\equiv\)N; (i) C₂H₅MgBr, (ii) H₃O⁺ gives [A]; the same nitrile with (i) CH₃MgBr, (ii) H₂O gives [B].)

Hint:

A Grignard reagent converts a nitrile into a ketone after hydrolysis; draw the ketone and count the carbons bearing four different groups.

Answer 1

Correct Answer: 3

The substrate is 3-methylcyclohexyl-CH(CH₃)-C\(\equiv\)N (chain and ring methyl are 1,3 on the ring).
\(\mathrm{R{-}C{\equiv}N + CH_3MgBr \rightarrow R{-}C(CH_3){=}NMgBr \xrightarrow{H_2O} R{-}CO{-}CH_3}\) (ketone).
So [B] = \(\mathrm{CH_3{-}CO{-}CH(CH_3){-}(3\text{-methylcyclohexyl})}\).
Chiral centres: (1) the side-chain carbon CH(CH₃), bonded to H, CH₃, COCH₃ and the ring: chiral. (2) Ring carbon C1 carrying the side chain: its two ring paths (towards C2 and towards C6) differ because of the methyl at C3: chiral. (3) Ring carbon C3 carrying the methyl: its two ring paths differ: chiral.
Number of chiral centres = 3.
Answer: 3